Magnetic recording medium

ABSTRACT

The magnetic recording medium includes a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an isoelectric point of a surface zeta potential of the magnetic layer is equal to or smaller than 3.8.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/009,570 filed Jun. 15, 2018, which claims priority under 35 U.S.C 119to Japanese Patent Application No. 2017-123041 filed on Jun. 23, 2017.The above applications are hereby expressly incorporated by reference,in their entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium.

2. Description of the Related Art

A magnetic recording medium including a magnetic layer includingferromagnetic powder and a binding agent on a non-magnetic support iscalled a coating type magnetic recording medium and is widely used asrecording medium for various purposes (for example, see JP1997-190623A(JP-H09-190623A).

SUMMARY OF THE INVENTION

In recent years, a magnetic recording medium such as a magnetic tapeused for data storage may be used in a low temperature and low humidityenvironment (for example, in an environment of a temperature of 10° C.to 15° C. and relative humidity of approximately 10% to 20%) such as adata center or the like, in which a temperature and humidity arecontrolled. Meanwhile, from a viewpoint of reducing air conditioningcost for controlling a temperature and humidity, it is desirable thatthe controlling conditions of a temperature and humidity during use canbe further alleviated than current conditions or it is desirable to makethe control unnecessary.

In consideration of such circumstances, the inventors have made studiesso as to alleviate controlling conditions of a temperature and humidityduring use of a magnetic recording medium or to make the controllingunnecessary. As a result, it is determined that, in an environment inwhich controlling conditions of a temperature and humidity arealleviated or the controlling thereof is not necessary (hereinafter,referred to as “in a high temperature and high humidity environment”),in a case of reproducing information recorded on a magnetic recordingmedium, a frequency of generation of a partial decrease in reproducingsignal amplitude (referred to as “missing pulse”) increases. The hightemperature and high humidity environment is an environment in which anatmosphere temperature is 30° C. to 45° C. and relative humidity isequal to or greater than 65% (for example, 65% to 90%). As thegeneration frequency of the missing pulse increases, an error rateincreases and reliability of a magnetic recording medium isdeteriorated. Accordingly, in order to use a magnetic recording mediumwith high reliability in the high temperature and high humidityenvironment, it is desired to decrease the generation frequency of themissing pulse in the high temperature and high humidity environment.

Therefore, an object of the invention is to provide a magnetic recordingmedium in which generation frequency of missing pulse in the hightemperature and high humidity environment is decreased.

According to one aspect of the invention, there is provided a magneticrecording medium comprising: a non-magnetic support; and a magneticlayer including ferromagnetic powder and a binding agent on thenon-magnetic support, in which an isoelectric point of a surface zetapotential of the magnetic layer is equal to or smaller than 3.8.

In one aspect, the isoelectric point may be 2.5 to 3.8.

In one aspect, the binding agent may be a binding agent including anacidic group.

In one aspect, the acidic group may include at least one kind of acidicgroup selected from the group consisting of a sulfonic acid group and asalt thereof.

In one aspect, the magnetic recording medium may further comprise anon-magnetic layer including non-magnetic powder and a binding agentbetween the non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a backcoating layer including non-magnetic powder and a binding agent on asurface side of the non-magnetic support opposite to a surface sideprovided with the magnetic layer.

According to one aspect of the invention, it is possible to provide amagnetic recording medium in which generation frequency of missing pulsein the high temperature and high humidity environment is decreased.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the invention relates to a magnetic recording mediumincluding: a non-magnetic support; and a magnetic layer includingferromagnetic powder and a binding agent on the non-magnetic support, inwhich an isoelectric point of a surface zeta potential of the magneticlayer is equal to or smaller than 3.8.

Hereinafter, the magnetic recording medium will be described morespecifically.

Magnetic Layer

Isoelectric Point of Surface Zeta Potential of Magnetic Layer

In the magnetic recording medium, the isoelectric point of the surfacezeta potential of the magnetic layer is equal to or smaller than 3.8. Inthe invention and the specification, the isoelectric point of thesurface zeta potential of the magnetic layer is a value of pH, in a casewhere a surface zeta potential of the magnetic layer measured by a flowpotential method (also referred to as a flow current method) becomeszero. A sample is cut out from the magnetic recording medium which is ameasurement target, and the sample is disposed in a measurement cell sothat the surface of the magnetic layer which is a target surface forobtaining the surface zeta potential comes into contact with anelectrolyte. Pressure in the measurement cell is changed to flow theelectrolyte and a flow potential at each pressure is measured, and then,the surface zeta potential is obtained by the following calculationexpression.

$\begin{matrix}{\zeta = {\frac{dI}{dp} \times \frac{\eta}{{ɛɛ}_{0}}\frac{L}{A}}} & \left( {{Calculation}\mspace{14mu}{Expression}} \right)\end{matrix}$

[ζ: surface zeta potential, p: pressure, I: flow potential, η: viscosityof electrolyte, ε: relative dielectric constant, ε₀: dielectric constantin a vacuum state, L: length of channel (flow path between twoelectrodes), A: area of cross section of channel]

The pressure is changed in a range of 0 to 400,000 Pa (0 to 400 mbar).The calculation of the surface zeta potential by flowing the electrolyteto the measurement cell and measuring a flow potential is performed byusing electrolytes having different pH (from pH of 9 to pH of 3 atinterval of approximately 0.5). A total number of measurement points is13 from the measurement point of pH 9 to the 13th measurement points ofpH 3. By doing so, the surface zeta potentials of each measurement pointof pH is obtained. As pH decreases, the surface zeta potentialdecreases. Thus, two measurement points at which polarity of the surfacezeta potential changes (a change from a positive value to a negativevalue) may appear, while pH decreases from 9 to 3. In a case where suchtwo measurement points appear, pH, in a case where the surface zetapotential is zero, is obtained by interpolation by using a straight line(linear function) showing a relationship between the surface zetapotential and pH of each of the two measurement points. Meanwhile, in acase where all of the surface zeta potentials obtained during thedecrease of pH from 9 to 3 is positive value, pH, in a case where thesurface zeta potential is zero, is obtained by extrapolation by using astraight line (linear function) showing a relationship between thesurface zeta potential and pH of the 13th measurement point (pH of 3)which is the final measurement point and the 12th measurement point. Onthe other hand, in a case where all of the surface zeta potentialsobtained during the decrease of pH from 9 to 3 is negative value, pH, ina case where the surface zeta potential is zero, is obtained byextrapolation by using a straight line (linear function) showing arelationship between the surface zeta potential and pH of the firstmeasurement point (pH of 9) which is the initial measurement point andthe 12th measurement point. By doing so, the value of pH, in a casewhere the surface zeta potential of the magnetic layer measured by theflow potential method is zero, is obtained.

The above measurement is performed three times in total at roomtemperature by using different samples cut out from the same magneticrecording medium (magnetic recording medium which is a measurementtarget), and pH, in a case where the surface zeta potential of eachsample is zero, is obtained. For the viscosity and the relativedielectric constant of the electrolyte, a measurement value at roomtemperature is used. The room temperature is set as 20° C. to 27° C. Anarithmetical mean of three pHs obtained as described above is anisoelectric point of the surface zeta potential of the magnetic layer ofthe magnetic recording medium which is a measurement target. Inaddition, as the electrolyte having pH of 9, an electrolyte obtained byadjusting pH of a KCl aqueous solution having a concentration of 1mmol/L to 9 by using a KOH aqueous solution having a concentration of0.1 mol/L is used. As the electrolyte having other pH, an electrolyteobtained by adjusting pH of the electrolyte having pH of 9, which isadjusted as described above, by using an HCl aqueous solution having aconcentration of 0.1 mol/L is used.

The isoelectric point of the surface zeta potential measured by themethod described above is an isoelectric point obtained regarding thesurface of the magnetic layer, unlike an isoelectric point offerromagnetic powder included in a magnetic layer disclosed inJP1997-190623A (JP-1109-190623A), for example. As a result of theintensive studies, the inventors have newly found that, by setting theisoelectric point of the surface zeta potential of the magnetic layer tobe equal to or smaller than 3.8, it is possible to decrease a generationfrequency of a missing pulse, in a case of reproducing informationrecorded on the magnetic recording medium in the high temperature andhigh humidity environment. In regards to this point, the inventors havesurmised as follows. However, the following description is merely asurmise, and the invention is not limited thereto.

It is thought that scraps (may be referred to as debris) are generateddue to chipping of the surface of the magnetic layer due to the contactbetween the surface of the magnetic layer and a reproducing head, in acase of reproducing information recorded on the magnetic recordingmedium. The inventors have surmised that, in a case where the scraps arestrongly stuck to the surface of the magnetic layer, a contact statebetween the surface of the magnetic layer and the reproducing head isnot stabilized, thereby partially decreasing a reproducing signalamplitude (missing pulse). With respect to this, the inventors havethought that, in a case of using the magnetic layer in which theisoelectric point of the surface zeta potential is in a region of acidicpH equal to or smaller than 3.8, a repulsive force may be easily appliedbetween the scraps generated from this magnetic layer and the surface ofthe magnetic layer. The inventors have surmised that preventing thescraps from being strongly stuck to the surface of the magnetic layer bythis repulsive force may contribute to a decrease in generation ofmissing pulse.

From a viewpoint of further decreasing the generation frequency of themissing pulse, the isoelectric point of the surface zeta potential ofthe magnetic layer is preferably equal to or smaller than 3.7, morepreferably equal to or smaller than 3.6, even more preferably equal toor smaller than 3.5, still more preferably equal to or smaller than 3.4,and still even more preferably equal to or smaller than 3.3.

As will be described later in detail, the isoelectric point of thesurface zeta potential of the magnetic layer can be controlled by thekind of a component used for forming the magnetic layer, a formationstep of the magnetic layer, and the like. From a viewpoint ofavailability of a component (for example, binding agent) used forforming the magnetic layer, the isoelectric point of the surface zetapotential of the magnetic layer is preferably equal to or greater than2.5, more preferably equal to or greater than 2.6, and even morepreferably equal to or greater than 2.7.

However, the invention is not limited to the above surmise and othersurmises described in the specification.

Next, the magnetic layer will be described more specifically.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic recordingmedium. From this viewpoint, ferromagnetic powder having an averageparticle size equal to or smaller than 50 nm is preferably used as theferromagnetic powder. Meanwhile, the average particle size of theferromagnetic powder is preferably equal to or greater than 10 nm, froma viewpoint of stability of magnetization.

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. An average particlesize of the ferromagnetic hexagonal ferrite powder is preferably 10 nmto 50 nm and more preferably 20 nm to 50 nm, from a viewpoint ofimprovement of recording density and stability of magnetization. Fordetails of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726Acan be referred to, for example.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. An average particle size ofthe ferromagnetic metal powder is preferably 10 nm to 50 nm and morepreferably 20 nm to 50 nm, from a viewpoint of improvement of recordingdensity and stability of magnetization. For details of the ferromagneticmetal powder, descriptions disclosed in paragraphs 0137 to 0141 ofJP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can bereferred to, for example.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so that the total magnification of 500,000 to obtain animage of particles configuring the powder. A target particle is selectedfrom the obtained image of particles, an outline of the particle istraced with a digitizer, and a size of the particle (primary particle)is measured. The primary particle is an independent particle which isnot aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of the plurality ofparticles not only includes an aspect in which particles configuring theaggregate are directly in contact with each other, but also includes anaspect in which a binding agent or an additive which will be describedlater is interposed between the particles. A term “particles” is alsoused for describing the powder.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph of 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. The components other than the ferromagnetic powder ofthe magnetic layer are at least a binding agent and one or more kinds ofadditives may be randomly included. A high filling percentage of theferromagnetic powder in the magnetic layer is preferable from aviewpoint of improvement recording density.

Binding Agent and Curing Agent

The magnetic recording medium is a coating type magnetic recordingmedium and includes a binding agent in the magnetic layer. The bindingagent is one or more kinds of resin. As the binding agent, variousresins normally used as a binding agent of a coating type magneticrecording medium can be used. For example, as the binding agent, a resinselected from a polyurethane resin, a polyester resin, a polyamideresin, a vinyl chloride resin, an acrylic resin obtained bycopolymerizing styrene, acrylonitrile, or methyl methacrylate, acellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin,and a polyvinylalkylal resin such as polyvinyl acetal or polyvinylbutyral can be used alone or a plurality of resins can be mixed witheach other to be used. Among these, a polyurethane resin, an acrylicresin, a cellulose resin, and a vinyl chloride resin are preferable.These resins may be homopolymers or copolymers. These resins can be usedas the binding agent even in the non-magnetic layer and/or a backcoating layer which will be described later.

For the binding agent described above, description disclosed inparagraphs 0028 to 0031 of JP2010-24113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 to 200,000 as a weight-average molecular weight. Theweight-average molecular weight of the invention and the specificationis a value obtained by performing polystyrene conversion of a valuemeasured by gel permeation chromatography (GPC) under the followingmeasurement conditions. The weight-average molecular weight of thebinding agent shown in examples which will be described later is a valueobtained by performing polystyrene conversion of a value measured underthe following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one aspect, as the binding agent, a binding agent including an acidicgroup can be used. The acidic group of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and a salt thereof. Specific examples of the acidicgroup include a sulfonic acid group, a sulfate group, a carboxyl group,a phosphate group, and a salt thereof. For example, a salt of sulfonicacid group (—SO3H) is represented by —SO3M, and M represents a grouprepresenting an atom (for example, alkali metal atom or the like) whichmay be cations in water or in an aqueous solvent. The same applies toaspects of salts of various groups described above. As an example of thebinding agent including the acidic group, a resin including at least onekind of acidic group selected from the group consisting of a sulfonicacid group and a salt thereof (for example, a polyurethane resin or avinyl chloride resin) can be used. However, the resin included in themagnetic layer is not limited to these resins. In addition, in thebinding agent including the acidic group, a content of the acidic groupcan be, for example, 0.03 to 0.50 meq/g. The content of variousfunctional groups such as the acidic group included in the resin can beobtained by a well-known method in accordance with the kind of thefunctional group. The amount of the binding agent used in a magneticlayer forming composition can be, for example, 1.0 to 30.0 parts by masswith respect to 100.0 parts by mass of the ferromagnetic powder.

In regards to the controlling of the isoelectric point of the surfacezeta potential of the magnetic layer, the inventors have surmised thatformation of the magnetic layer so that the acidic component is unevenlydistributed to a surface portion of the magnetic layer contributes to adecrease in value of the isoelectric point. In addition, it is surmisedthat formation of the magnetic layer so as to decrease the amount of abasic component present in the surface portion of the magnetic layeralso contributes to a decrease in value of the isoelectric point. Theacidic component is used as a meaning including a state of a componentcapable of emitting H⁺ in water or an aqueous solvent to dissociateanions and a salt thereof. The basic component is used as a meaningincluding a state of a component capable of emitting OH⁻ in water or anaqueous solvent to dissociate cations and a salt thereof. For example,it is thought that, in a case of using the acidic component, unevendistribution of the acidic component to the surface portion of themagnetic layer contributes to a decrease in value of the isoelectricpoint of the surface zeta potential of the magnetic layer to control theisoelectric point to be equal to or smaller than 3.8. For example, theinventors have thought that, in a step of applying a magnetic layerforming composition onto a non-magnetic support directly or through anon-magnetic layer, the applying which is performed in an alternatingmagnetic field by applying an alternating magnetic field contributes toformation of a magnetic layer in which the acidic component is unevenlydistributed to the surface portion. As the acidic component, forexample, a binding agent including an acidic group can be used. Inaddition, the inventors have surmised that, in a case of using thebinding agent including an acidic group, in a preparation step of themagnetic layer forming composition, the addition (additional addition)of the binding agent even in a case of mixing magnetic liquid and othercomponents, after preparing a dispersion liquid (magnetic liquid)including ferromagnetic powder and the binding agent, contributes toformation of a magnetic layer in which the binding agent including theacidic group is unevenly distributed to the surface portion. Theformation of the magnetic layer will be described later morespecifically.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneaspect, a thermosetting compound which is a compound in which a curingreaction (crosslinking reaction) proceeds due to heating can be used,and in another aspect, a photocurable compound in which a curingreaction (crosslinking reaction) proceeds due to light irradiation canbe used. At least a part of the curing agent is included in the magneticlayer in a state of being reacted (crosslinked) with other componentssuch as the binding agent, by proceeding the curing reaction in themagnetic layer forming step. This point is the same as regarding a layerformed by using a composition, in a case where the composition used forforming the other layer includes the curing agent. The preferred curingagent is a thermosetting compound, polyisocyanate is suitable. Fordetails of the polyisocyanate, descriptions disclosed in paragraphs 0124and 0125 of JP2011-216149A can be referred to, for example. The amountof the curing agent can be, for example, 0 to 80.0 parts by mass withrespect to 100.0 parts by mass of the binding agent in the magneticlayer forming composition, and is preferably 50.0 to 80.0 parts by mass,from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and the binding agent,and may include one or more kinds of additives, if necessary. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive included in the magnetic layerinclude non-magnetic powder (for example, inorganic powder or carbonblack), a lubricant, a dispersing agent, a dispersing assistant, anantibacterial agent, an antistatic agent, and an antioxidant. As thenon-magnetic powder, non-magnetic powder which can function as anabrasive, non-magnetic powder (for example, non-magnetic colloidparticles) which can function as a projection formation agent whichforms projections suitably protruded from the surface of the magneticlayer, and the like can be used. An average particle size of colloidalsilica (silica colloid particles) shown in the examples which will bedescribed later is a value obtained by a method disclosed in ameasurement method of an average particle diameter in a paragraph 0015of JP2011-048878A. As the additives, a commercially available productcan be suitably selected according to the desired properties ormanufactured by a well-known method, and can be used with any amount. Asan example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic recordingmedium may include a magnetic layer directly on the surface of thenon-magnetic support or may include a magnetic layer on the surface ofthe non-magnetic support directly or indirectly through the non-magneticlayer including the non-magnetic powder and the binding agent. Thenon-magnetic powder used in the non-magnetic layer may be inorganicpowder or organic powder. In addition, carbon black and the like can beused. Examples of the inorganic powder include powder of metal, metaloxide, metal carbonate, metal sulfate, metal nitride, metal carbide, andmetal sulfide. These non-magnetic powder can be purchased as acommercially available product or can be manufactured by a well-knownmethod. For details thereof, descriptions disclosed in paragraphs 0146to 0150 of JP2011-216149A can be referred to. For carbon black which canbe used in the non-magnetic layer, descriptions disclosed in paragraphs0040 and 0041 of JP2010-24113A can be referred to. The content (fillingpercentage) of the non-magnetic powder of the non-magnetic layer ispreferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

In the invention and the specification, the non-magnetic layer alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to asa “support”) will be described. As the non-magnetic support, well-knowncomponents such as polyethylene terephthalate, polyethylene naphthalate,polyamide, polyamide imide, aromatic polyamide subjected to biaxialstretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heat treatmentmay be performed with respect to these supports in advance.

Back Coating Layer

The magnetic recording medium can also include a back coating layerincluding non-magnetic powder and a binding agent on a surface side ofthe non-magnetic support opposite to the surface side provided with themagnetic layer. The back coating layer preferably includes any one orboth of carbon black and inorganic powder. In regards to the bindingagent included in the back coating layer and various additives which canbe randomly included therein, a well-known technology regarding thetreatment of the magnetic layer and/or the non-magnetic layer can beapplied.

Various Thicknesses

A thickness of the non-magnetic support is preferably 3.00 to 20.00 μm,more preferably 3.00 to 10.00 μm, and even more preferably 3.00 to 6.00μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount of a magnetic head used, a head gaplength, a recording signal band, and the like. The thickness of themagnetic layer is normally 0.01 μm to 0.15 μm, and is preferably 0.02 μmto 0.12 μm and more preferably 0.03 μm to 0.10 μm from a viewpoint ofrealization of high-density recording. The magnetic layer may be atleast one layer, or the magnetic layer can be separated to two or morelayers having magnetic properties, and a configuration regarding awell-known multilayered magnetic layer can be applied. A thickness ofthe magnetic layer which is separated into two or more layers is a totalthickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand is preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and even more preferably 0.10 to 0.70 μm.

The thicknesses of various layers of the magnetic recording medium andthe non-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magneticrecording medium in a thickness direction is, for example, exposed by awell-known method of ion beams or microtome, and the exposed crosssection is observed with a scanning electron microscope. In the crosssection observation, various thicknesses can be acquired as a thicknessacquired at one portion of the cross section in the thickness direction,or an arithmetical mean of thicknesses acquired at a plurality ofportions of two or more portions, for example, two portions which arerandomly extracted. In addition, the thickness of each layer may beacquired as a designed thickness calculated according to themanufacturing conditions.

Manufacturing Method of Magnetic Recording Medium

Each composition for forming the magnetic layer, and the non-magneticlayer and the back coating layer which are randomly provided, normallyincludes a solvent, together with various components described above. Asthe solvent, various organic solvents generally used for manufacturing acoating type magnetic recording medium can be used. The amount of thesolvent in each layer forming composition is not particularly limited,and can be set to be the same as that of each layer forming compositionof a typical coating type magnetic recording medium. Steps of preparingthe composition for forming each layer generally include at least akneading step, a dispersing step, and a mixing step provided before andafter these steps, if necessary. Each step may be divided into two ormore stages. All of raw materials used in the invention may be added atan initial stage or in a middle stage of each step. In addition, eachraw material may be separately added in two or more steps. For example,in the preparation of the magnetic layer forming composition, thebinding agent including an acidic group can be separately added throughtwo or more steps. It is preferable that a dispersion liquid is preparedby mixing some components including the ferromagnetic powder among thevarious components of the magnetic layer forming composition with thebinding agent including an acidic group and dispersing the mixture in asolvent, and the binding agent including an acidic group is also addedin a step of mixing the dispersion liquid with the remaining componentsand performing dispersing, because it is possible to contribute to thecontrolling of the isoelectric point of a surface zeta potential of themagnetic layer to be equal to or smaller than 3.8. In addition, it ispreferable that the non-magnetic powder which can function as anabrasive dispersed separately from the ferromagnetic powder, and thenother components such as the ferromagnetic powder is mixed anddispersed, in order to improve dispersibility of the ferromagneticpowder and the non-magnetic powder (abrasive).

In order to prepare each layer forming composition, a well-knowntechnology can be used. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. The details ofthe kneading processes of these kneaders are disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-79274A (JP-H01-79274A). In addition, inorder to disperse each layer forming composition, as a dispersionmedium, at least one or more kinds of dispersion beads selected from thegroup consisting of glass beads and other dispersion beads can be used.As such dispersion beads, zirconia beads, titania beads, and steel beadswhich are dispersion beads having high specific gravity are suitable.These dispersion beads are preferably used by optimizing a particlediameter (bead diameter) and a filling percentage. As a dispersingmachine, a well-known dispersing machine can be used. Each layer formingcomposition may be filtered by a well-known method before performing thecoating step. The filtering can be performed by using a filter, forexample. As the filter used in the filtering, a filter having a holediameter of 0.01 to 3 μm (for example, filter made of glass fiber orfilter made of polypropylene) can be used, for example.

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the surface of the non-magnetic support orperforming multilayer coating with the non-magnetic layer formingcomposition in order or at the same time. For details of the coating forforming each layer, a description disclosed in a paragraph 0066 ofJP2010-231843A can be referred to. In addition, the coating of themagnetic layer forming composition performed in an alternating magneticfield can contribute to the controlling of the isoelectric point of asurface zeta potential of the magnetic layer to be equal to or smallerthan 3.8. The inventors have surmised that this is because, an acidiccomponent (for example, the binding agent including an acidic group) iseasily unevenly distributed to a surface portion of a coating layer ofthe magnetic layer forming composition due to the applied alternatingmagnetic field, and thus, by drying this coating layer, a magnetic layerin which the acidic component is unevenly distributed to the surfaceportion is obtained. However, this is merely a surmise. The applying ofthe alternating magnetic field can be performed by disposing a magnet ina coating device so that the alternating magnetic field is appliedvertically to the surface of the coating layer of the magnetic layerforming composition. A magnetic field strength of the alternatingmagnetic field can be, for example, set as approximately 0.05 to 3.00 T.However, there is no limitation to this range. The “vertical” in theinvention and the specification does not mean only a vertical directionin the strict sense, but also includes a range of errors allowed in thetechnical field of the invention. For example, the range of errors meansa range of less than ±10° from an exact vertical direction.

For various other steps for manufacturing the magnetic recording medium,a well-known technology can be applied. For details of the varioussteps, descriptions disclosed in paragraphs 0067 to 0070 ofJP2010-231843A can be referred to, for example. It is preferable thatthe coating layer of the magnetic layer forming composition is subjectedto an alignment process, while the coating layer is wet (not dried). Forthe alignment process, various well-known technologies such as adescription disclosed in a paragraph 0067 of JP2010-231843A can be used.In a case of performing the alignment process, it is preferable to applya magnetic field (for example, DC magnetic field) for aligning theferromagnetic powder with respect to the coating layer of the magneticlayer forming composition applied in the alternating magnetic field.

As described above, it is possible to obtain the magnetic recordingmedium according to one aspect of the invention. The magnetic recordingmedium can be a tape-shaped magnetic recording medium (magnetic tape) orcan also be a disk-shaped magnetic recording medium (magnetic disk). Forexample, the magnetic tape is normally accommodated in a magnetic tapecartridge and the magnetic tape cartridge is mounted in a magnetic tapedevice (referred to as a drive). A servo pattern can also be formed inthe magnetic tape by a well-known method, in order to allow headtracking servo to be performed in the drive. The controlling conditionsof a temperature and humidity of an environment of the drive in a caseof reproducing information recorded on the magnetic tape in the driveare alleviated or made unnecessary to be controlled, and thus, even in acase where the reproducing is performed in the high temperature and highhumidity environment, it is possible to decrease the generationfrequency of the missing pulse, as long as it is the magnetic recordingmedium according to one aspect of the invention.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description are based on mass.

A “binding agent A” described below is a SO₃Na group-containingpolyurethane resin (weight-average molecular weight: 70,000, SO₃Nagroup: 0.20 meq/g).

A “binding agent B” described below is a vinyl chloride copolymer(product name: MR110, SO₃K group-containing vinyl chloride copolymer,SO₃K group: 0.07 meq/g) manufactured by Kaneka Corporation.

Manufacturing of Magnetic Tape

Example 1

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a SO₃Na group-containingpolyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g)manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent ofmethyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solventwere mixed with 100.0 parts of alumina powder (HIT-80 manufactured bySumitomo Chemical Co., Ltd.) having a gelatinization ratio of 65% and aBrunauer-Emmett-Teller (BET) specific surface area of 20 m²/g, anddispersed in the presence of zirconia beads by a paint shaker for 5hours. After the dispersion, the dispersion liquid and the beads wereseparated by a mesh and an alumina dispersion was obtained.

(2) Magnetic Layer Forming Composition List

Magnetic Liquid

Ferromagnetic powder: 100.0 parts

-   -   Ferromagnetic hexagonal barium ferrite powder having average        particle size (average plate diameter) of 21 nm

Binding agent (see Table 1): see Table 1

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Solution

Alumina dispersion prepared in the section (1): 6.0 parts

Silica Sol (projection forming agent liquid)

Colloidal silica (Average particle size: 120 nm) 2.0 parts

Methyl ethyl ketone: 1.4 parts

Other Components

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Polyisocyanate (CORONATE (registered trademark) manufactured by TosohCorporation): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(3) Non-Magnetic Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   Average particle size (average long axis length): 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

Binding agent A: 18.0 parts

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

(4) Back Coating Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

-   -   Average particle size (average long axis length): 0.15 μm    -   Average acicular ratio: 7    -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

Sulfonic acid group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Methyl ethyl ketone: 155.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

(5) Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

The magnetic liquid was prepared by dispersing (beads-dispersing) eachcomponent by using a batch type vertical sand mill for 24 hours.Zirconia beads having a bead diameter of 0.5 mm were used as thedispersion beads.

The prepared magnetic liquid and abrasive solution, the binding agent A(0.1 parts with respect to 100.0 parts of the ferromagnetic powder ofthe magnetic liquid) to be additionally added, and other components(silica sol, other components, and finishing additive solvent) weremixed with each other and beads-dispersed for 5 minutes by using thesand mill, and the treatment (ultrasonic dispersion) was performed witha batch type ultrasonic device (20 kHz, 300 W) for 0.5 minutes. Afterthat, the obtained mixed solution was filtered by using a filter havinga hole diameter of 0.5 μm, and the magnetic layer forming compositionwas prepared.

The non-magnetic layer forming composition was prepared by the followingmethod.

Each component excluding the lubricant (stearic acid, stearic acidamide, and butyl stearate), cyclohexanone, and methyl ethyl ketone wasdispersed by using batch type vertical sand mill for 24 hours to obtaina dispersion liquid. As the dispersion beads, zirconia beads having abead diameter of 0.5 mm were used. After that, the remaining componentswere added into the obtained dispersion liquid and stirred with adissolver. The dispersion liquid obtained as described above wasfiltered with a filter having a hole diameter of 0.5 μm and anon-magnetic layer forming composition was prepared.

The back coating layer forming composition was prepared by the followingmethod.

Each component excluding polyisocyanate and cyclohexanone was kneaded byan open kneader and diluted, and was subjected to a dispersion processof 12 passes, with a transverse beads mill disperser and zirconia beadshaving a bead diameter of 1 mm, by setting a bead filling percentage as80 volume %, a circumferential speed of rotor distal end as 10 m/sec,and a retention time for 1 pass as 2 minutes. After that, the remainingcomponents were added into the obtained dispersion liquid and stirredwith a dissolver. The dispersion liquid obtained as described above wasfiltered with a filter having a hole diameter of 1 μm and a back coatinglayer forming composition was prepared.

(6) Manufacturing Method of Magnetic Tape

The non-magnetic layer forming composition prepared in the section (5)was applied to a surface of a support made of polyethylene naphthalatehaving a thickness of 5.00 μm so that the thickness after the dryingbecomes 1.00 μm and was dried to form a non-magnetic layer.

Then, in a coating device disposed with a magnet for applying analternating magnetic field, the magnetic layer forming compositionprepared in the section (5) was applied onto the surface of thenon-magnetic layer so that the thickness after the drying becomes 0.10μm, while applying an alternating magnetic field (magnetic fieldstrength: 0.15 T), to form a coating layer. The applying of thealternating magnetic field was performed so that the alternatingmagnetic field was applied vertically to the surface of the coatinglayer. After that, a homeotropic alignment process was performed byapplying a magnetic field having a magnetic field strength of 0.30 T ina vertical direction with respect to a surface of a coating layer, whilethe coating layer of the magnetic layer forming composition is wet (notdried). After that, the coating layer was dried to form a magneticlayer.

After that, the back coating layer forming composition prepared in thesection (5) was applied to the surface of the support made ofpolyethylene naphthalate on a side opposite to the surface where thenon-magnetic layer and the magnetic layer were formed, so that thethickness after the drying becomes 0.50 μm, and was dried to form a backcoating layer.

After that, a surface smoothing treatment (calender process) wasperformed by using a calender roll configured of only a metal roll, at aspeed of 100 m/min, linear pressure of 294 kN/m (300 kg/cm), and acalender temperature (surface temperature of a calender roll) of 100° C.

Then, the heat treatment was performed in the environment of theatmosphere temperature of 70° C. for 36 hours. After the heat treatment,the slitting was performed to have a width of ½ inches (0.0127 meters),and a servo pattern was formed on the magnetic layer by a commerciallyavailable servo writer.

By doing so, a magnetic tape of Example 1 was manufactured.

Examples 2 to 6 and Comparative Examples 1 to 8

A magnetic tape was manufactured by the same method as in Example,except that various conditions were changed as shown in Table 1.

In Table 1, in the examples and the comparative examples in which“performed” is shown in the column of the additional addition of thebinding agent, the additional addition of the binding agent A (0.1 partswith respect to 100.0 parts of the ferromagnetic powder of the magneticliquid) was performed in the same manner as in Example 1. On the otherhand, in the comparative examples in which “not performed” is shown inthis column, the additional addition of the binding agent A was notperformed.

In Table 1, in the examples and the comparative examples in which“performed” is shown in the column of the alternating magnetic fieldapplication during coating, the step subsequent to the coating step ofthe magnetic layer forming composition was performed by the same methodas in Example 1. That is, the application of the alternating magneticfield was performed during coating of the magnetic layer formingcomposition in the same manner as in Example 1. On the other hand, inthe comparative examples in which “not performed” is shown in thiscolumn, the step subsequent to the coating step of the magnetic layerforming composition was performed by the same method as in Example 1,except that the application of the alternating magnetic field was notperformed.

Evaluation of Physical Properties of Magnetic Tape

(1) Isoelectric Point of Surface Zeta Potential of Magnetic Layer

Six samples for isoelectric point measurement were cut out from eachmagnetic tape of the examples and the comparative examples and disposedin the measurement cell of two samples in one measurement. In themeasurement cell, a sample installing surface and a surface of the backcoating layer of the sample were bonded to each other by using adouble-sided tape in upper and lower sample table (size of each sampleinstalling surface is 1 cm×2 cm) of the measurement cell. Accordingly,in a case where an electrolyte flows in the measurement cell, thesurface of the magnetic layer of the sample comes into contact with theelectrolyte, and thus, the surface zeta potential of the magnetic layercan be measured. The measurement was performed three times in total byusing two samples in each measurement, and the isoelectric points of thesurface zeta potential of the magnetic layer were obtained. Anarithmetical mean of the obtained three values was shown in Table 1, asthe isoelectric point of the surface zeta potential of the magneticlayer of each magnetic tape. As a surface zeta potential measurementdevice, SurPASS manufactured by Anton Paar was used. The measurementconditions were set as follows. Other details of the method of obtainingthe isoelectric point is as described above.

Measurement cell: variable gap cell (20 mm×10 mm)

Measurement mode: Streaming Current

Gap: approximately 200 μm

Measurement temperature: room temperature

Ramp Target Pressure/Time: 400,000 Pa (400 mbar)/60 seconds

-   Electrolyte: KCl aqueous solution having concentration of 1 mmol/L    (adjusted pH to 9)

pH adjusting solution: HCl aqueous solution having concentration of 0.1mol/L or KOH aqueous solution having concentration of 0.1 mol/L

Measurement pH: pH 9→pH 3 (measured at 13 measurement points in total atinterval of approximately 0.5

(2) Missing Pulse Generation Frequency

The following measurement was performed in the high temperature and highhumidity environment of a temperature of 32° C. and relative humidity of80%.

A magnetic tape cartridge accommodating each magnetic tape (magnetictape total length of 500 m) of the examples and the comparative exampleswas set in a drive of Linear Tape-Open Generation 6 (LTO-G6)manufactured by IBM, and the magnetic tape was subjected toreciprocating running 1,500 times at tension of 0.6 N and a runningspeed of 8 m/sec.

The magnetic tape cartridge after the running was set in a referencedrive (LTO-G6 drive manufactured by IBM), and the magnetic tape isallowed to run to perform the recording and reproducing. A reproducingsignal during the running was introduced to an external analog/digital(AD) conversion device. A signal having a reproducing signal amplitudewhich is decreased 70% or more than an average (average of measuredvalues at each track) was set as a missing pulse, a generation frequency(number of times of the generation) thereof was divided by the totallength of the magnetic tape to obtain a missing pulse generationfrequency (unit: times/m) per unit length (per 1 m) of the magnetictape. In a case where the missing pulse generation frequency is equal toor smaller than 5.0 times/m, the magnetic tape can be determined as amagnetic tape having high reliability in practice.

The results of the above evaluation are shown in Table 1 (Table 1-1 andTable 1-2).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Formation of Content of binding Binding agent A 5 parts 10 parts 15 pans20 parts  0 parts 10 parts magnetic agent in magnetic Binding agent B 0parts  0 parts  0 parts  0 parts 10 parts 10 parts layer liquidAdditional addition of binding agent Performed Performed PerformedPerformed Performed Performed Alternating magnetic field applicationPerformed Performed Performed Performed Performed Performed duringcoating Isoelectric point of surface zeta potential of magnetic layer3.8 3.6 3.2 2.7 3.5 3.0 Missing pulse generation frequency (times/m) 3.53.1 2.8 2.0 3.3 3.0

TABLE 1-2 Comparative Comparative Comparative Comparative ComparativeComparative Comparative Comparative Example 1 Example 2 Example 3Example 4 Example 5 Example 6 Example 7 Example 8 Formation of Contentof Binding 5 parts 10 parts 15 parts 20 parts 15 parts 15 parts  0 parts10 parts magnetic binding agent A layer agent in Binding 0 parts  0parts  0 parts  0 parts  0 parts  0 parts 10 parts 10 parts magneticagent B liquid Additional addition Not Not Not Not Performed Not Not Notof binding agent performed performed performed performed performedperformed performed Alternating magnetic Not Not Not Not Not PerformedNot Not field application performed performed performed performedperformed performed performed during coating Isoelectric point ofsurface zeta  5. 0 4.8 4.6 4.6 4.2 4.6 4.8 4.7 potential of magneticlayer Missing pulse generation 12.5 6.2 5.8 5.7 5.4 5.8 6.0 5.9frequency (times/m)

From the results shown in Table 1, in the magnetic tapes of the examplesin which the isoelectric point of the surface zeta potential of themagnetic layer is equal to or smaller than 3.8, it is possible toconfirm that the generation frequency of the missing pulse in the hightemperature and high humidity environment is small, compared to that inthe magnetic tapes of the comparative examples.

One aspect of the invention is effective in a technical field of variousmagnetic recording medium such as a magnetic tape for data storage.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; and a magnetic layer including ferromagneticpowder and a binding agent on the non-magnetic support, wherein anaverage particle size of the ferromagnetic powder is 10 nm to 21 nm, andan isoelectric point of a surface zeta potential of the magnetic layeris equal to or smaller than 3.8.
 2. The magnetic recording mediumaccording to claim 1, wherein the isoelectric point is 2.5 to 3.8. 3.The magnetic recording medium according to claim 1, wherein the bindingagent is a binding agent including an acidic group.
 4. The magneticrecording medium according to claim 3, wherein the acidic group includesat least one kind of acidic group selected from the group consisting ofa sulfonic acid group and a salt thereof.
 5. The magnetic recordingmedium according to claim 2, wherein the binding agent is a bindingagent including an acidic group.
 6. The magnetic recording mediumaccording to claim 5, wherein the acidic group includes at least onekind of acidic group selected from the group consisting of a sulfonicacid group and a salt thereof.
 7. The magnetic recording mediumaccording to claim 1, further comprising: a non-magnetic layer includingnon-magnetic powder and a binding agent between the non-magnetic supportand the magnetic layer.
 8. The magnetic recording medium according toclaim 1, further comprising: a back coating layer including non-magneticpowder and a binding agent on a surface side of the non-magnetic supportopposite to a surface side provided with the magnetic layer.